[S-VC31] Active Volcanism

[SVC31-P01] Magma ascent and oxidation process of caldera-forming eruption: Insight from textural and synchrotron analyses for 7.6ka Mashu eruption

*Kyohei Sano¹, Tesuo Harada², Keiji Wada³, Eiichi Sato³ (1.Graduate School of Regional Resource Management, University of Hyogo, 2.Laboratory of Advanced Science and Technology for Industry, University of Hyogo, 3.Earth Science Laboratory, Course of Subjects Education, Graduate School of Education, Hokkaido University of Education at Asahikawa)

Keywords:magma oxidation, pumice, texture

Caldera forming eruption has a huge influence on human society and it is important to reveal the sequence of magma ascent process during the eruption. Approximately 7.6 ka, caldera-forming activity of Mashu Volcano started as phreatomagmatic eruption (Ma-j) and followed Plinian falls (Ma-i, -h, -g) and pyroclastic flow (Ma-f). In this study, we performed the textural and synchrotron analyses for the pumice from Mashu caldera-forming activity and discuss the magma ascent and oxidation processes during the caldera-forming eruption.
We conducted the textural and synchrotron analyses for Mashu caldera-forming eruption. We sieved the samples and picked up the pumices larger than 2φ. Samples were washed by a sonic wave washer and dried at 110℃. Powdered pumices were used for the synchrotron analysis. To determine the Fe³⁺/Fe²⁺ ratio, we collected the absorption spectra between 650 [eV] and 750 [eV] with using the standard materials at room temperature using the BL-10 beamline at the New SUBARU, Laboratory of Advanced Science and Technology for Industry, University of Hyogo, Japan. The soft X-rays were focused on 2.5 mm × 1.2 mm area. In the Ma-i layer, Fe³⁺/Fe²⁺ changes from 11.5 to 16.6 from lower to the upper Ma-i pumice. On the other hand, Fe³⁺/Fe²⁺ in Ma-h layer changes from 9.2 to 13.1. Furthermore, we analyzed the vesicle texture by EPMA and estimate the magma ascent rate. We discuss the magma ascent and oxidation process of caldera-forming eruption based on the textural and synchrotron analyses for the Ma pumices.